A dental curable composition containing a polymerizable monomer, a filler, and a polymerization initiator is called a composite resin, and this dental material is most widely used today as a restorative material for repairing fractures of teeth and dental caries. Such a dental curable composition is required to have the following properties. Specifically, as a cured product obtained after polymerization curing, the dental curable composition is required to have sufficient mechanical strength and hardness to serve as a substitute for natural teeth, wear resistance to biting pressure in an oral cavity, surface smoothness and gloss, etc. Furthermore, as a paste which has not yet been polymerized and cured, the dental curable composition is expected to offer ease of handling (or to have excellent handling properties) for dental clinicians and technicians, for example, proper fluidity and forming property, no adhesion to dental instruments, no stickiness, etc.
These properties of the dental curable composition are greatly influenced by the material, shape, particle size, and content of a filler used therein and by the combination of fillers used together. For example, when an inorganic filler having an average particle size of more than 1 μm is used, the filling rate of the filler in the polymerizable monomer can be increased easily and therefore sufficient mechanical strength as a cured product and excellent handling properties as a paste can be obtained. The use of such an inorganic filler has, however, a drawback in that it is difficult to obtain satisfactory gloss even after final polishing, and even if satisfactory gloss is obtained, the gloss cannot be retained for a long time. On the other hand, when an inorganic ultrafine particle filler having an average particle size of 1 μm or less is used, the surface smoothness and gloss after polishing of the cured product and the gloss durability in the oral cavity are improved. The use of such an inorganic ultrafine particle filler has, however, a drawback in that when the inorganic filler is mixed and kneaded with the polymerizable monomer, the viscosity of the resulting paste increases significantly, which makes it difficult to increase the content of the filler. As a result, the mechanical strength of the cured product decreases, and the unpolymerized pasty composition becomes sticky, which reduces the handling properties. Under these circumstances, it is difficult to increase the mechanical strength and the surface smoothness and gloss after polishing of the cured product and the handling properties of the paste in a balanced manner.
On the other hand, dental curable compositions are required not only to have the above-mentioned properties but also to match natural teeth, that is, to have optical properties such as color, transparency, and light diffusion similar to those of natural teeth. Particularly in recent years, there has been a demand for dental curable compositions having optical properties similar to those of dentin or optical properties similar to those of enamel so that they can match respective parts of a tooth to be restored. In other words, dental curable compositions having different colors, transparencies, and light diffusions are expected to be used for different parts. It is possible to adjust the color and transparency by adjusting the content of a pigment, but it is difficult to adjust the light diffusion only by adjusting the content of the pigment.
Patent Literature 1 describes a dental paste containing at least 55% by weight in total of porous, non-pyrogenic silica and non-aggregated primary silica particles having an average diameter of at most about 200 nm. More specifically, one example discloses a dental paste containing, as fillers, non-aggregated silica particles having an average particle size of about 75 nm and aggregated non-pyrogenic silica obtained by spray-drying silica sol having an average particle size of about 75 nm to aggregate the particles.
Patent Literature 2 describes a dental composite material containing a polymerizable monomer, one kind of filler, and another kind of filler. The former filler is obtained by aggregating inorganic filler particles having an average particle size of 0.01 to 1 μm and subjecting the resulting aggregated particles to heat treatment so that the primary particles are loosely bonded with each other, and has a refractive index difference of 0.06 or less from the cured polymerizable monomer. The latter filler has a refractive index difference of more than 0.06 from the cured polymerizable monomer, and has an average particle size of 1 μm or more. This dental composite material has a degree of diffusion D of 0.002 to 0.3, when it is represented by the following formula (1):D=(I20/cos 20°+I70/cos 70°/(2I0)  (1)where I denotes the intensity of transmitted light through a sample, and I0, I20 and I70 denote the intensities of the transmitted light measured at angles of 0, 20, and 70 degrees respectively with respect to the direction perpendicular to the sample plate (the incident direction of the light). One example of Patent Literature 2 discloses a dental composite material containing a polymerizable monomer, one kind of filler, and another kind of filler. The former filler is obtained by aggregating barium boroaluminosilicate glass having an average particle size of 0.7 μm and calcining the resulting aggregated particles so that the primary particles are loosely bonded with each other, and has a refractive index of 1.55. The latter filler is obtained by aggregating spherical silica having an average particle size of 0.2 μm and calcining the resulting aggregated particles.
Patent Literature 3 describes a dental curable composition containing a polymerizable monomer, and an organic-inorganic composite filler having a refractive index difference of 0.01 or more as an absolute value from the cured polymerizable monomer and having an average particle size of 1 to 20 μm. In this dental curable composition, the degree of diffusion D represented by the above formula (1) has a value of 0.01 or more.
CITATION LISTPatent Literature 1JP 2005-517688 TPatent Literature 2JP 09(1997)-255516 APatent Literature 3JP 2002-138008 A